1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and particularly, to an LCD device capable of improving a viewing angle characteristic by employing a plurality of biaxial optical films.
2. Background of the Invention
Recently, the development of various types of portable electric equipment, such as mobile phones, personal digital assistants (PDAs), and note book computers, is increasing the demands on flat panel display devices which are applicable to those equipment and small in size, light in weight and power-efficient. Examples of the flat panel display device are a liquid crystal display (LCD) device, a plasma display panel (PDP) device, a field emission display (FED) device, a vacuum fluorescent display (VFD) device and the like. Studies on those devices are actively conducted. Among others, the LCD device is currently in the limelight in view of its mass production technology, facilitation of driving scheme and implementation of high color rendering property.
Such LCD device implements various display modes according to alignment of liquid crystal molecules. However, TN mode LCD device is usually used in recent time by virtue of facilitation of white and black color rendering, fast response speed and low driving voltage. In the TN mode LCD device, when a voltage is applied, liquid crystal molecules having aligned parallel with a substrate are, re-aligned almost perpendicularly to the substrate, which occurs a problem that a viewing angle becomes narrower when a voltage is applied due to a refractive anisotropy of the liquid crystal molecules.
To overcome the problem of the viewing angle, various modes of LCD devices having a wide viewing angle characteristic have been introduced in recent time. Among others, an In-Plane Switching (IPS) Mode LCD device has actually undergone for mass-production. The IPS-mode LCD device is configured such that at least a pair of electrodes aligned parallel with each other are formed in each pixel so as to create a transversal field actually parallel with a substrate, thus aligning liquid crystal molecules on a plane surface.
FIG. 1 is a view showing an IPS-mode LCD device according to the related art, wherein FIG. 1A is a plane view thereof and FIG. 1B is a sectional view taken along the line I-I′ of FIG. 1A.
As shown in FIG. 1A, pixels of a liquid crystal display panel 1 are defined by gate lines 3 and data lines 4 arranged in horizontal and vertical directions. The drawings merely show (n,m)th pixel; however, n gate lines 3 and m data lines 4 are actually arranged on the liquid crystal display panel 1 so as to define n×m pixels on the whole liquid crystal display panel 1. A thin film transistor (TFT) 10 is formed at an intersection between the gate line 3 and the data line 4 in each pixel. The TFT 10 includes a gate electrode 11 to which a scan signal is applied via the gate line 3, a semiconductor layer 12 formed on the gate electrode 11 and activated responsive to the applied scan signal so as to form a channel layer, and a source electrode 13 and a drain electrode 14 formed on the semiconductor layer 12 and to which an image signal is applied via the data line 4. The thusly-constructed TFT 10 applies the image signal input from the exterior into a liquid crystal layer.
A plurality of common electrodes 5 and pixel electrodes 7 arranged to be substantially parallel with the data lines 4. Also, a common line 16 connected to the common electrodes 5 is disposed at the center of each pixel. A pixel electrode line 18 connected to the pixel electrodes 7 is disposed on the common line 16 so as to overlap the common line 16. As the common line 16 and the pixel electrode line 18 overlap each other, a storage capacitance is generated in the IPS-mode LCD device.
As such, in the IPS-mode LCD device having such construction, liquid crystal molecules are aligned to be substantially parallel with the common electrodes 5 and the pixel electrodes 7. When a signal is applied to the pixel electrodes 7 in cooperation with the TFTs 10 being driven, a transversal field substantially parallel with the liquid crystal display panel 1 is generated between the common electrodes 5 and the pixel electrodes 7. The liquid crystal molecules rotate on the same level along the transversal field, thereby preventing a gray scale inversion due to the refractive anisotropy thereof.
Hereinafter, the related art IPS-mode LCD device having such construction will be described in more detail with reference to FIG. 1b. 
As shown in FIG. 1B, the gate electrode 11 is formed on a first substrate 20. A gate insulating layer 22 is laminated all over the first substrate 20. A semiconductor layer 12 is formed on the gate insulating layer 22, and the source electrode 13 and the drain electrode 14 are formed on the semiconductor layer 12. A passivation layer 24 is formed all over the first substrate 20. A first alignment layer 28a having a decided alignment direction for aligning liquid crystal molecules by a rubbing or the like, is formed on the passivation layer 24.
Further, a plurality of common electrodes 5 are formed on the first substrates 20, and the pixel electrode 7 and the data line 4 are formed on the gate insulating layer 22, thereby generating a transversal field E between the common electrode 5 and the pixel electrode 7.
A black matrix 32 and a color filter layer 34 are formed on a second substrate 30. The black matrix 32 serves to prevent light leakage into an area in which liquid crystal molecules are not operated. As shown in the drawings, the black matrix 32 is usually formed at a region of the TFT 10 and between pixels (that is, a region of gate line and data line). The color filter layer 34 is provided to render actual colors with red (R), green (G) and blue (B) colors. An overcoat layer 36 for protecting the color filter layer 34 and improving flatness of a substrate is formed on the color filter layer 34, and a second alignment layer 28b having decided alignment direction is formed on the overcoat layer 36.
A liquid crystal layer 40 is formed between the first substrate 20 and the second substrate 30, so as to completely form the liquid crystal display panel 1.
As aforementioned, in the IPS-mode LCD device, the transversal field is generated within the liquid crystal layer 40 by the common electrodes 5 and the pixel electrodes 7 respectively formed on the first substrate 20 and the gate insulating layer 22, accordingly the liquid crystal molecules within the liquid crystal layer 40 are rotated on the same level, thereby preventing the gray scale inversion due to the refractive anisotropy of the liquid crystal molecules.
However, the IPS-mode LCD device has the following problems. That is, in the IPS-mode LCD device, the liquid crystal molecules are rotated on the same level along the transversal field, so as to prevent the gray scale inversion due to the refractive anisotropy of the liquid crystal molecules and thusly improve a viewing angle characteristic in a vertical (up-and-down) direction or a horizontal (right-and-left) direction whereas not improving the viewing angle characteristic in a diagonal direction of a screen.